Mouse glomerular epithelial cells previously characterized have been investigated for biosynthesis of proteoglycans. Confluent monolayers of epithelial cells were radiolabeled. It was found that these cells produced Heparan Sulfate basement membrane in high amounts. Analysis of immunoprecipitates showed both a mature proteoglycan and a precursor core protein band. These proteoglycans are an important determinant of the permselectivity properties of the glomerular basement membrane.